Various types of magnetic head are known for writing and/or reading magnetic signals that correspond to digital data.
In the simplest embodiment, a magnetic head comprises a magnetic circuit or core of magnetic material, e.g. ferrite. The core is U-shaped and the ends of its two branches or arms meet across a gap in which a separator of non-magnetic material is positioned.
A winding is wound around the portion of the core that interconnects the two branches (the portion furthest from ne gap) or else around one of the branches. The winding may also be distributed over both branches.
One terminal of the winding is connected to the electronic read and/or write circuit and the other terminal of the winding is put to ground potential or is connected to another portion of the electronic circuit. For example, when writing, an electrical signal issued by the electronic write circuit travels through the winding, thereby generating a corresponding longitudinal magnetic field.
The magnetic field is channelled in the core. The leakage field created around the gap causes data to be recorded on a magnetic medium placed close to the gap.
When reading, the magnetic flux from the medium is diverted into the magnetic circuit and gives rise to electrical signal in the winding. The electrical signal is amplified and processed by the electronic read circuit.
This type of embodiment gives rise to electrical problems. That is why another type of magnetic head is used, as shown diagrammatically in FIG. 1.
Such known magnetic heads have a first winding 10 wound around one of the branches 12 of the U-shaped core, and a second winding 14 wound around the other branch 16 in identical manner to the first winding. These two windings are electrically interconnected at a midpoint 18 with the winding directions of the windings being such that a current flowing through the first winding towards the midpoint generates a magnetic field of the same intensity and in the opposite direction to the magnetic field generated by the same current flowing through the second winding, likewise towards the midpoint.
To obtain the desired result, i.e. to solve the above-mentioned electrical problems, the midpoint or connection 18 where the first winding 10 is connected to the second winding 14 is put to ground potential. Each winding is connected to a respective electronic read and/or write circuit (not shown) and is used independently of the other.
For example, when writing digital signals constituted by 0s and 1s, the first winding 10 is used for writing 0s whereas the second winding 14 is used for writing 1s, with the opposite-direction magnetic fields generated by the windings 10 and 14 corresponding to 0s and to 1s. The windings 10 and 14 are decoupled by the common midpoint 18 being put to ground potential.
Although a device of the above type is satisfactory from the electrical point of view, it suffers from drawbacks from the magnetic point of view.
In devices of that type, when a current flows through one or other of the windings, the magnetic field generated is not completely channelled within the core: magnetic leakage occurs, radiating outwards from the winding in question.
The intensity of this leakage radiation lies in the range 1/10000-th to 1/1000-th of the intensity of the magnetic field channelled by the magnetic circuit. Although weak, it is nevertheless sufficient to be detected by other magnetic heads placed in the vicinity thereof.
For example, FIG. 2 is a diagram of a turntable 20 having magnetic head supports 22 disposed at the periphery thereof. Devices of this type are used in "rotating head" type read/write mechanisms. Magnetic heads 24 dedicated to writing alternate with magnetic heads 26 dedicated to reading, and are therefore in the immediate vicinity thereof.
The distance between two magnetic heads 24 and 26 is of the order of a few millimeters to a few centimeters. Thus the leakage radiation emitted by a winding while writing can be read by an adjacent read head, thereby giving rise to crosstalk phenomena.
In known devices, to avoid such disturbing leakage radiation, each of the magnetic heads of a magnetic read/write mechanism is placed in a metal box (not shown in FIG. 2) that provides screening.
It will be understood that this prior art solution includes its own drawbacks. The screening box is an additional component that requires additional handling while it is being installed. The physical size of each head is considerably increased, and this can be highly disadvantageous in devices where maximum miniaturization is required. Finally, the metal screening box increases the cost of a magnetic head.